Vehicle engines may be configured to shut-off during idle conditions when the vehicle comes to a stop while a brake is applied and restarted once the brake is released (e.g., a stop/start system). Such stop/start systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like. Fuel consumption may be further reduced by shutting down the engine before braking, for example, during extended coasting periods.
One example approach to shut-down and subsequently restart the engine while the vehicle is traveling is disclosed by Ries-Muller in U.S. Pat. No. 6,951,525. Therein, when the vehicle is moving with the operator foot off the accelerator and the brake pedal not applied (that is, the vehicle is coasting), the engine is shut-down and the transmission clutches are disengaged to interrupt the engine braking torque. During a subsequent restart, the engine is restarted prior to a transition from free-wheel mode to engaged clutch travel mode by employing the fuel injection system using a charge regulator and/or an electric motor. After the engine is restarted, the reference describes using a combination of a braking intervention and a clutch intervention to maintain a vehicle braking torque during the transition, and thereby reduce undesirable vehicle deceleration when the vehicle speed is below a threshold.
However, in this approach, when the vehicle is restarted while the vehicle is still moving, the combination of the wheel torque from the moving vehicle and the engine torque from the spinning engine may lead to a torque surge and the vehicle may lurch forward when a clutch is suddenly engaged. As such, this may degrade the quality of the restart.
Thus in one example, some of the above issues may be addressed by a method for controlling a vehicle engine, the engine being coupled to a stepped gear ratio transmission. In one embodiment, the method comprises, in response to a first vehicle moving condition, shutting down the engine and at least partially disengaging the transmission while the vehicle is moving; and during a subsequent restart, while the vehicle is moving, starting the engine using starter motor assistance and adjusting a degree of engagement of a transmission clutch to adjust a torque transmitted to a wheel of the vehicle. The method may further comprise, in response to a second vehicle moving condition, not shutting down the engine.
In one example, the first vehicle moving condition may include a coasting condition wherein the vehicle is moving and the vehicle operator has not depressed the accelerator pedal or the brake pedal, and the vehicle speed is above a threshold. In response to the coasting condition, the engine may be shutdown, for example, by shutting off a fuel supply to the engine. Then, the vehicle transmission may be at least partially disengaged. In one example, the transmission may be fully disengaged. In another example, the transmission may be partially disengaged and a transmission clutch may be slipped controllably. For example, a smaller amount of clutch slippage may be provided, the amount adjusted responsive to the vehicle speed and/or engine speed at the time of engine shutdown.
During a subsequent restart, while the vehicle is still moving, and with the transmission clutch still engaged, the engine may be started by activating a starter motor, and a degree of engagement of the transmission clutch may be adjusted, for example by engaging the clutch and controllably slipping the clutch, to adjust a torque transmitted to the wheel. For example, a larger amount of clutch slippage may be provided, the amount adjusted responsive to the vehicle speed and/or engine speed at the time of engine restart.
In this way, an engine may be restarted with starter motor assistance while the vehicle is still moving, and without transmitting the engine torque to the wheels. By reducing the amount of engine torque transferred to the wheels and added to the wheel torque, a smoother transition between engine combusting and non-combusting modes may be achieved. Further, potential vehicle lurches from a sudden clutch engagement may also be reduced. As such, this may substantially improve the quality of engine restarts. Similarly, by reducing the amount of wheel torque that is transmitted from the wheels of a coasting vehicle to the engine following engine shutdown, a faster engine spin-down may be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.